Add Devise case-insensitive keys integration specs
Add Devise case-insensitive keys integration

See https://github.com/plataformatec/devise/blob/v3.4.1/lib/generators/templates/devise.rb#L45-L48
Refactor extract method `integrate_with_devise_case_insensitive_keys`

Closes #137
Merge branch 'add-devise-case-insensitive-keys-integration'